Neuromuscular disorders comprise a heterogeneous group of clinical conditions that primarily affect one or more components of the neuromuscular unit — typically skeletal muscle — but are often also multisystemic. The considerable clinical impact of these diseases is exemplified by the muscular dystrophies and by spinal muscular atrophy (SMA), for which the development for novel molecular therapies is both urgent and challenging.
Recent advances in RNA biology have accelerated the progress of a new generation of molecular therapies based on RNA. Single- or double-stranded nucleic acid agents and small-molecule agents are being developed as novel therapies to target the mutant mRNAs that are involved in neuromuscular disease. These approaches modulate pre-mRNA processing or inhibit the deleterious effects of toxic RNAs.
In the case of Duchenne muscular dystrophy (DMD), exon-skipping therapies to restore a viable open reading frame in the DMD gene are well advanced. Two recent systemic delivery clinical trials have reported encouraging data on the restoration of dystrophin protein expression, and second-generation compounds are in development. An alternative approach to stimulate the readthrough of premature stop codons, which is applicable to a subset of patients with DMD, is also in development.
Myotonic dystrophy arises as a result of an expanded microsatellite repeat mutation that leads to a gain-of-function toxic mRNA, which ultimately causes muscle degeneration and multisystem dysfunction. Various approaches to correct the deleterious toxic effects of toxic RNA using small-molecule-based, oligonucleotide-based or RNA interference-based methods to inhibit or degrade the toxic RNA are being investigated.
A viable therapeutic strategy for SMA, which arises as a result of the loss-of-function of the survival of motor neuron 1 (SMN1) gene, is to restore the splicing of exon 7 in the related SMN2 gene, which has the potential to fully compensate for the loss of the SMN1 protein. Recent studies using oligonucleotide-mediated exon-inclusion methods appear to be highly promising.
Although the development of RNA-based therapies for neuromuscular disease remains challenging, recent progress in this field is encouraging. However, major barriers remain the poor in vivo delivery of most RNA therapeutic agents and the regulatory hurdles that are associated with the development of novel personalized medicines.
The development of effective therapies for neuromuscular disorders such as Duchenne muscular dystrophy (DMD) is hampered by considerable challenges: skeletal muscle is the most abundant tissue in the body, and many neuromuscular disorders are multisystemic conditions. However, despite these barriers there has recently been substantial progress in the search for novel treatments. In particular, the use of antisense oligonucleotides, which are designed to target RNA and modulate pre-mRNA splicing to restore functional protein isoforms or directly inhibit the toxic effects of pathogenic RNAs, offers great promise and these approaches are now being tested in the clinic. Here, we review recent advances in the development of such antisense oligonucleotides and other promising novel approaches, including the induction of readthrough nonsense mutations.
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F.M. is supported by the Great Ormond Street Children's Charity. The financial support of the Medical Research Council (MRC), the Muscular Dystrophy Campaign, the French Muscular Dystrophy Association (AFM), the Wellcome Trust and TREAT-NMD is also gratefully acknowledged. M.J.A.W. is supported by the MRC, the AFM, the Wellcome Trust, Parkinson's UK, Action Duchenne, the Muscular Dystrophy Campaign, Duchenne Ireland and Muscular Dystrophy Ireland. The authors wish to thank Q. Lu, Neuromuscular/ALS Center, Carolinas Medical Center, Charlotte, North Carolina, USA, for providing Figure 2.
Francesco Muntoni has previously served on the scientific advisory boards for AVI BioPharma, he has a patent pending regarding tailed antisense oligonucleotides to redirect splicing, and his university receives research support from AVI BioPharma and GlaxoSmithKline in relation to clinical trials on antisense oligonucleotides.
Matthew J. A. Wood has a patent pending regarding peptides for the delivery of antisense oligonucleotides.
- Neuromuscular disease
A disease affecting one or more constituent components of the neuromuscular unit, which comprise motor neurons, neuromuscular synapses and skeletal muscle.
- Duchenne muscular dystrophy
(DMD). A severe, X-linked neuromuscular disease that arises as a result of mutations in the dystrophin gene that abolish the open reading frame, leading to the absence of dystrophin protein.
- Myotonic dystrophy
The most common cause of muscular dystrophy in adults, which arises as a result of microsatellite repeat expansion mutations in the myotonin protein kinase gene (leading to muscular dystrophy type 1) or the cellular nucleic acid-binding protein gene (leading to muscular dystrophy type 2). These mutations lead to the formation of directly pathogenic RNAs and widespread dysregulation of splicing.
- Spinal muscular atrophy
(SMA). A very common childhood genetic disease that arises as a result of loss-of-function mutations in the survival of motor neuron 1 gene, which lead to motor neuron degeneration and subsequent axial and limb muscle weakness.
- Pre-mRNA splicing
The processing of a pre-mRNA transcript to remove specific intronic and some exonic sequences that are not required for the generation of the mature mRNA.
- RNA-targeted therapies
Therapeutic modalities that are based on the targeted modification or inhibition of RNAs (for example, pre-mRNA, mature mRNA or non-coding RNA) that are directly implicated in disease.
- Antisense oligonucleotide
A short single-stranded nucleic acid, typically 15–25 nucleotides in length, that has the ability to mediate therapeutic effects by directly interacting with pre-mRNA or mRNA in a sequence-specific manner.
- 2′-O-methyl RNA
An antisense oligonucleotide in which the RNA backbone is modified by the addition of methyl (CH3) groups at the 2′ position in the ribose ring.
- Phosphorodiamidate morpholino
(PMO). An extensively modified antisense oligonucleotide that is neutrally charged and comprises a morpholino ring-based RNA backbone.
- Open reading frame
(ORF). A stretch of a DNA sequence that does not contain a stop or termination codon, and typically corresponds to the legible DNA sequence of a single gene.
- Exon skipping
The processing of a pre-mRNA transcript to result in the exclusion of a specific exon within the mature mRNA transcript.
- mdx mouse
A naturally occurring mouse mutant that carries a premature termination codon in exon 23 of the dystrophin (Dmd) gene, which results in the absence of dystrophin protein.
- Premature termination codon
A mutation that results in the presence of a de novo termination or stop codon in advance of where one would normally be found, and which serves to disrupt the open reading frame for that gene.
- Cell-penetrating peptides
Short peptide fragments that are able to mediate enhanced cellular uptake of a range of cargoes by virtue of their cationic charge and the presence of specific cationic amino acid residues, typically arginine residues.
An arginine-rich cell-penetrating peptide comprising eight arginine residues and the non-natural amino acid X (6-aminohexanoic acid).
(PPMO). A phosphorodiamidate morpholino (PMO) oligonucleotide to which a short peptide fragment is attached at either the 5′ or 3′ end by direct chemical conjugation.
- Stop codon readthrough
A therapeutic method of addressing the deleterious effect of a premature termination codon in which the readthrough of a premature termination codon is stimulated in order to yield a full-length open reading frame.
- Microsatellite repeat expansion
The presence of repeat units of a microsatellite sequence in the genome, which expand beyond the number of repeat units that are associated with normal health, thus leading to a mutation that is directly pathogenic.
A disease that arises through widespread dysregulation of pre-mRNA splicing.
- Exon inclusion
The processing of a pre-mRNA transcript to result in the inclusion of a specific exon within the mature mRNA transcript.
- Personalized molecular therapies
The development of therapeutic agents that are directly tailored to specific mutations or to individual patients or subsets of patients.
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Cite this article
Muntoni, F., Wood, M. Targeting RNA to treat neuromuscular disease. Nat Rev Drug Discov 10, 621–637 (2011). https://doi.org/10.1038/nrd3459
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